Introduction to Polymers Introduction to Polymers Poly = many,
mer = unit, many units Polymer science is relatively a new branch
of science. It deals with chemistry physics and mechanical
properties of macromolecule.
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A polymer is a large molecule which is formed by repeated
linking of the small molecules called monomers. polymer is organic
substance made up of many repeating units or building blocks of
molecules called mers. OR
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POLYMER Combine, many monomers to create a polymer. Polymer is
often used as a synonym for plastic. All plastic are polymers, but
not all polymers are plastics. Poly mers are made up of many Mono
mer Many Units One Unit
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Monomer molecules joined in units of long polymer.
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It consist of large no. of repeating units known as monomers
The no. of repeating units in a chain of polymer is known as degree
of polymerization
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POLYMER a family of natural and synthetic materials made of
repetition of high weight molecules in a form of flexible chain
NATURAL POLYMER Collagen Gelatin Silk Wool Natural rubber DNA
SYNTHETIC POLYMER Polyethylene terephthalate (PET) High Density
Polyethylene (HDPE) Polyvinyl Chloride (PVC) Low Density
Polyethylene (LPDE) Polypropylene (PP) Polystyrene (PS)
Characteristics of Polymers Low Density. Low coefficient of
friction. Good corrosion resistance. Good mould ability. Excellent
surface finish can be obtained. Can be produced with close
dimensional tolerances. Economical. Poor tensile strength. Low
mechanical properties. Poor temperature resistance. Can be produced
transparent or in different colours.
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Properties of Polymers The physical properties of a polymer,
such as its strength and flexibility depend on: Chain length - in
general, the longer the chains the stronger the polymer; Side
groups - polar side groups give stronger attraction between polymer
chains, making the polymer stronger; Branching - straight, un
branched chains can pack together more closely than highly branched
chains, giving polymers that are more crystalline and therefore
stronger; Cross-linking - if polymer chains are linked together
extensively by covalent bonds, the polymer is harder and more
difficult to melt.
Classification of Polymer is based on source structure
Polymerization Molecular force
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Classification based on Source Natural polymers Semi-synthesis
polymers Synthesis polymers
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Natural polymers The definition of a natural polymer is a
polymer that results from only raw materials that are found in
nature. Example:- Proteins, Cellulose, Starch, Rubber.
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Semi-synthesis polymers chemically treated polymers of natural
origin are quite common and of great practical importance
Cellulose, for example, is used in two different ways: it is
dissolved using some special solvent and precipitated again in a
different physical shape, e.g. viscose silk (reyon) copper silk
chemically treated polymers, that are of natural origin termed as
semi synthesis
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Synthesis polymers Synthetic polymers are derived from
petroleum oil, and made by scientists and engineers. Examples of
synthetic polymers include nylon, polyethylene, polyester, Teflon,
and epoxy.
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Based on structure Linear polymers Branched chain polymers
Cross linked chain polymer
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Linear polymers consists of a long string of carbon-carbon
bonds misleading because the geometry around each carbon atom is
tetrahedral and the chain is neither linear nor straight As the
polymer chain grows, it folds back on itself in a random fashion
E.g Polyethylene
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Branched chain polymers Polymers with branches at irregular
intervals along the polymer chain are called branched polymers
difficult for the polymer molecules to pack in a regular array less
crystalline and less dense amount and type of branching also
affects physical properties such as viscosity and elasticity
Branches often prevent chains from getting close enough together
for intermolecular forces to work effectively. E.g. polyethylene PE
is a relatively simple polymer, there are two common forms of it
(high and low-density,) each with unique properties.
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Cross linked chain polymers formed from bi-functional and tri-
functional monomers and contain strong covalent bonds contain short
side chains (cross links) connect different polymer chains into a
network adding cross-links between polymer chains makes the polymer
more elastic (they can stretch and return to their original form)
Cross links between chains
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Based on polymerization additionalcondensation
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Addition polymers formed by the repeated addition of monomer
molecules possessing double or triple bonds n(CH2=CH2) -(CH2 -CH2
)- Ethylene polyethylene
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Condensation polymers formed by repeated condensation reaction
between two different bi- functional or tri-functional monomeric
units. eg. terylene (dacron), nylon 6, 6, nylon 6. n(H2N(CH2)6 NH2)
+ n(HOOC(CH2)4COOH) [-NH(CH2)6NHCO(CH2)4CO-]n + nH2O (Nylon
6:6)
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Based on molecular force nylonthermoplasticthermosetting
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Nylon Nylon is used as general name for all synthetic fiber
forming polyamides, i.e., having a protein like structure. These
are the condensation polymers of diamines and dibasic acids A
number is usually suffixed with the Nylon which refers to the
number of carbon atoms present in the diamine and the dibasic acids
respectively. example: nylon 6,6 nylon-6,6: Nylon-6,6 is obtained
by the polymerisation of adipic acid with hexamethylene
diamine.
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Thermoplastic polymers These are linear or slightly branched
long chain polymers, which can be softened on heating &
reversibly hardened on cooling repeatedly. Their hardness is a
temporary property & varies with temperature. Example:-
polyvinyl chloride. Polyvinyl chloride:- It is a vinyl polymer
constructed of repeating vinyl groups (ethenyls) having one of
their hydrogens replaced with a chloride group.
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Thermosetting polymers initial mixture of reactive, low molar
mass compounds reacts upon heating in the mold to form an
insoluble, infusible network. Example: bakelite bakelite: bakelite
is formed of phenol and form-aldehyde polymerization.
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Applications of Polymers: Polymeric materials are used in and
on soil to improve aeration, provide mulch, and promote plant
growth and health.
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Medicine Many biomaterials; heart valve replacements blood
vessels, are made of polymers like Dacron, Teflon and
polyurethane.
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Consumer Science Plastic containers of all shapes and sizes are
light weight and economically less expensive than the more
traditional containers. Clothing floor coverings garbage disposal
bags packaging are other polymer applications.
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Industry Automobile parts windshields for fighter planes Pipes
Tanks packing materials insulation, wood substitutes elastomers are
all polymer applications used in the industrial market.
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Sports Playground equipment various balls golf clubs swimming
pools protective helmets are often produced from polymers.
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Strength of Polymers In general, the longer the polymer chain,
the stronger the polymer. There are two reasons for this: longer
chains are more tangled there are more intermolecular forces
between the chains because there are more points of contact. These
forces, however, are quite weak for polyethene. Areas in a polymer
where the chains are closely packed in a regular way are said to be
crystalline. The percentage of crystallinity in a polymer is very
important in determining its properties. The more crystalline the
polymer, the stronger and less flexible it becomes.
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When a polymer is stretched (cold-drawn), a neck forms. In the
neck the polymer chains line up producing a more crystalline
region. Cold-drawing leads to an increase in strength. The first
polyethene which was made contained many chains which were
branched. This resulted in a relatively disorganised structure of
low strength and density. This was called low density polyethene
(ldpe). In the crystalline form, the methyl groups all have the
same orientation along the chain. This is called the isotactic
form. In the amorphous form, the methyl groups are randomly
orientated. This is called the atactic form. Polymers with a
regular structure are said to be stereoregular.
